1. Field of the Invention
This invention relates to a ballast for the pulsed-mode operation of gas discharge lamps. More specifically, it concerns the use of a self-excited or externally controlled half or full bridge circuit, likewise for the pulsed mode operation of gas discharge lamps.
2. Description of the Related Art
Conventional electronic ballasts (EVG) serve both to improve brightness control and to make extended and more sparing ignition of high and low pressure gas discharge lamps (GE) possible. Such electronic ballasts supply a frequency in the range of 20 kHz to 70 kHz to a series resonant load circuit. By changing the frequency of the output voltage of the EVG the load current in the series-resonant circuit can be adjusted continuously and in a stepless manner. The gas discharge lamp that is arranged in the load circuit and has the load current flowing through it can thus have its brightness changed in a stepless manner. At the same time, by the output frequency of the EVG approaching the resonant frequency of the load circuit when the lamp is not ignited, a high voltage can be generated at the lamp, to cause the lamp to ignite.
Whilst such EVG's enable comfortable brightness control and ignition, they are not equally suitable to alter the other characteristics of a gas discharge lamp, such as colour temperature or light output (valued luminous flux, lumen/watt).
This can now be achieved by operating a gas discharge lamp periodically with two dissimilar, substantially different current intensities. On the one hand a high current value is applied to the lamp for a certain period and on the other hand the lamp is allowed a recovery time with a substantially lower holding current.
This cycle is repeated periodically so that averaged over time it is ensured that the lamp remains in the conducting state and takes up the lamp nominal power rating P.sub.N. By changing the duration of the respective time intervals and the amplitude of the current pulses during each respective time interval the colour temperature and light output of the gas discharge lamps, in particular the Na-high pressure lamps, can be changed. To date, however, this change requires that to achieve different current intensities for the main pulse and the substantially lower holding current pulse the chokes arranged in the load circuit be switched over by additional switches. These switches are expensive and complicated.